This invention relates to endoprosthetic stents that are placed within body lumens that exhibit physiologic motion such as peristaltic motion.
Medical stents are tubular endoprostheses placed within the body to perform a function such as maintaining open a body lumen, for example, a passageway occluded by a tumor. Typically, the stent is delivered inside the body by a catheter that supports the stent in a compacted form as it is transported to the desired site. Upon reaching the site, the stent is expanded so that it engages the walls of the lumen. The expansion mechanism may involve forcing the stent to expand radially outward, for example, by inflation of a balloon carried by the catheter, to inelastically deform the stent and fix it at a predetermined expanded position in contact with the lumen wall. The expansion balloon can then be deflated and the catheter removed.
In another technique, the stent is formed of a highly elastic material that will self-expand after being compacted. During introduction into the body, the stent is restrained in the compacted condition. When the stent has been delivered to the desired site for implantation, the restraint is removed, allowing the stent to self-expand by its own internal elastic restoring force.
Strictures of the esophagus often produce obstructive dysphagia resulting in debilitating malnutrition. To date, the theoretical advantages of placing a plastic stent to restore the patient""s ability to swallow have been offset by technical difficulty of placement, morbidity and mortality associated with the procedure, and poor long-term prosthesis performance. In particular, previous stents have transmitted the force and deformation of peristaltic waves inappropriately, for instance causing the stent to creep toward the stomach, perforate the esophagus, or rupture the aorta.
In a first aspect, the invention features a method for providing reinforcement to the lumen of a peristaltic organ. The stent is formed by knitting a filament into interknit loops, the pattern of the loops selected such that from a relaxed state each row of loops may shift axially relative to and independently of the rows on either side. The local lengthening and shortening allowed by the shifting allows the stent to accommodate the peristalsis of the organ without migrating within the organ.
Preferred embodiments of the stent feature the following. The lumen treated is the esophagus. The elongation factor xcex5 by which the stent can locally lengthen by shifting is related to the angle xcex8 at which the lumen can incline inward by the relationship: xcex5=1.0/cos xcex8. The stent is knitted of metal wire to be self-expandable such that the stent expands outward against the body lumen wall by an elastic restoring force of the wire. The stent is knitted from nitinol wire having a diameter of about 0.15 mm. The stent, in its free state, has a point of constricted cross-section. The constriction may have a valve.
A stent according to the invention offers the following advantages. The stent exerts a constant, gentle radial force on the wall of the lumen that maintains lumen patency and actively resists compression, as by a tumor. The inherent flexibility of the knitted stent adapts to peristalsis, transmitting the peristaltic wave to the lumen, but without changing overall length or creeping. This reduces complications and promotes long-term stability, patency, and patient comfort. The force exerted by the stent against the lumen is sufficient to compress the capillaries of the organ so that growth into the lumen is retarded. The stent can be delivered via a low-profile delivery system which is smaller than a standard endoscope. The small diameter of the delivery system simplifies implantation by eliminating the need for pre-dilating the stricture, and allows placement even in patients with tortuous esophageal anatomy or strictures prone to perforation by plastic stents.
In a second aspect, the invention features a stent for providing reinforcement to a selected region of a selected body lumen. The stent comprises two resilient cylindrical mesh layers and a semi-permeable compliant membrane sandwiched between.
Preferred embodiments of the invention feature the following. The two mesh layers may be knit of a flexible filament, and the knit may be configured so that the stent can adapt to peristalsis of the body lumen. The membrane is composed of expanded polytetrafluoroethylene.
The invention or preferred embodiments thereof may feature the following advantages. The semi-permeable membrane prevents cell ingrowth of the stent. The force exerted by the stent against the lumen is sufficient to compress the capillaries of the organ so that growth into the lumen is retarded.
In a third aspect, the invention features a method of manufacturing a delivery system for a resilient tubular device so that the tubular device can be inserted into the body in a substantially reduced diameter. The method uses a confining block having a bore and a slot leading into the bore. The tubular device is pinched and inserted into the bore and the slot. Two mandrels are inserted into the bore, one inside the tubular device and one outside. The mandrels are revolved about each other to roll the tubular device on itself until the tubular device is entirely rolled and confined at the reduced diameter within the bore. The tubular device is removed from the bore while being restrained in the reduced diameter.
Preferred embodiments of the method of manufacture feature the following. The removing step may be accomplished by pushing the tubular device from the end of the bore and restraining the tubular device as it emerges. The restraining my be by means of wrapping a wire around the tubular device. The slot may be tangent to the bore of the confining block. The tubular device may be a stent knit of an elastic filament. One of the mandrels may be part of the delivery system that will be used to deliver the stent.
The inventive method of manufacturing the stent features the following advantages. Certain prior methods required several operators to simultaneously hold and constrain the resiliency of the stent and hurt the fingers of the operators. The method of the invention requires only one operator and is comfortable to execute. The stent delivery systems produced by the method are more uniform than those manufactured by previous methods, both in distribution of stresses within a single stent and in variation between stents, thus avoiding deformation of the stent during manufacture and allowing the physician to place the stent more precisely in the patient. A stent delivery system manufactured according to the method has a small profile, and thus minimizes trauma to the patient during implantation.
Other advantages and features of the invention will become apparent from the following description of a preferred embodiment, and from the claims.